Not applicable
1. Field of the Invention
The present invention relates to chromium dioxide. More particularly, the present invention relates to chromium dioxide powders.
2. General Background of the Invention
Half-metallic oxide CrO2 has long been of importance in magnetic recording and shows unique magnetic properties. In 1986, it was predicted to be half-metallic by Schwarz using band structure calculation [1]. A number of experiments have been taken and confirmed its half-metallicity, including photoemission experiment [2], superconducting point contact experiment [3], and vacuum tunneling measurement [4]. It is suggested that half-metallic ferromagnets are ideal materials for the electrodes in spin dependent tunneling devices, such as those described by Moodera et al. [5]. Since the spin polarization is nearly 100% at the Fermi level, the tunneling junctions made of such materials would have an extremely large magnetoresistance (MR) and a very significant switching effect. It has been reported that CrO2 thin films can have negative MR of about 13%-25% [6,7] at low temperature. In 1998, Manoharan et al. [8] and Coey et al. [9] studied the cold-pressed powder samples and found that the MR of the pressed compacts can reach as high as 30-50%. The conduction mechanism of these CrO2 powder compacts arises from the spin dependent intergranular tunneling influenced by the Coulomb gap [9,10]. A CrO2-I-Co magnetic tunneling junction using native oxide barrier has also been reported but with a small MR of about 1% [11].
The following U.S. Patent is incorporated herein by reference: U.S. Pat. No. 5,856,008. This patent discloses CrO2 powders NOT aligned in a magnetic field, while the powders of the present invention are aligned in a magnetic field. Their data show magnetoresistance of 12% at 5 Kelvin and 20 kOe. The aligned powders of the present invention show magnetoresistance of greater magnitude at much lower field (41% magnetoresistance at 5 Kelvin and 1 kOe). Low field and high magnetoresistance are required for this to be usable.
The present inventors have studied the magnetic and transport properties of aligned needle-shaped CrO2 powders. The powders are aligned to the same direction in a strong magnetic field and then affixed to a substrate. The sample is strongly anisotropic owing to the shape anisotropy of the particles. The spin dependent intergranular tunneling is the major conduction mechanism of the CrO2 aligned powders. A large negative MR of about 41% has been found at 5 K, and the magnetoresistance shows two well-separated peaks at the coercivity fields (+1000 Oe), which bears a resemblance to the behavior of the magnetic tunneling junctions.
It is suggested that the described switching characteristics of the aligned particles may have useful applications. Some possible applications are: magnetic random access memory (MRAM), which is faster or smaller than the RAMs currently in use, and most importantly, it is non-volatile (meaning it is not erased when one turns the computer off). In addition, the material of the present invention can be used as magnetic field sensors and used in spin-transistors.